This invention relates in general to alcohol detection devices and methods, and in particular to a new and useful method for determining the alcohol concentration of the blood by measuring the alcohol concentration of the breathing air of a person.
As a practical matter, it is not possible for a test person whose breath is being collected for alcohol detection to breathe over a longer period of time so shallowly that only the oral cavity and the upper respiratory tracts are ventilated. After about one minute, the lungs require a deep breath. The breathing air exhaled after that is then in equilibrium with the blood alcohol. A measurement with an alcohol-meter then indicates the alcohol content of the blood.
A known arrangement for determining the alcohol concentration of the blood measures the alcohol in the breathing air at a time determined by a time control. This time is determined by the expiration of time predetermined interval starting within the exhaling period. The breathing air throughout must not drop during this period below a given minimum throughput, and must only flow in the exhaling direction. When these two conditions are not met, an error detector determines the invalidity of the measurement.
The given time interval is provided to ensure that the test person has already exhaled the air from the oral cavity and the trachea at the time of measurement, and that the measuring instrument measures the alcohol concentration of the breathing air from the alveoli of the lungs. The expiration of the given time interval is determined by the time at which a minimum amount of breathing air, preferably at least 75% of the total breathing air volume, has been exhaled. An integrator can integrate the breathing air throughout in time during inhalation and exhalation, and determine therefrom the expiration of the time interval after the breathing air minimum volume. The embodiment is to be independent of the physical structure of the test person who may either have a big lung or may not be cooperative. A deliberately flat inhalation may simulate a much too low breathing capacity. The automatically established minimum breathing volume can then in the practical test only be mixed air from the lungs and the oral and larygneal cavity (West German Offenlegungsschrift No. 24 28 353).
Another known method and the respective device or arrangement therefore start from the fact that the true alcohol concentration in the breathing air is only determined if that portion of the exhaled air is tested for its alcohol value that could establish in the alveoli of the lungs equilibrium with the alcohol concentration of the blood. The shuttle air between oral and laryngeal cavity and the mixed air from the alveolar air must therefore, be measured separately.
The method and the respective arrangement solve this problem by means of an infrared measuring instrument which constantly measures the alcohol concentration during the sampling.
A threshold comparator determines the variation of the measured values per unit of time which is a measure for the rate of rise of the alcohol concentration.
A measured value is only transmitted to the indicator when the rate of rise drops below a given threshold value. This first condition results from the fact that the portion of the shuttle air from the oral and laryngeal cavity diminishes constantly with a drop in the rate of rise, and only alveolar air is contained in the measuring channel of the device when it drops below the threshold value. Another condition for transmitting the measured value is that the rate of flow of the exhaling air determined by a flowmeter must have been above a given value during a given time period until the measuring value is transmitted. This second condition measures the provided course of the measuring method. The alcohol concentration is measured by an infrared measuring instrument with a short response time arranged in the breathing air current.
The device is voluminous and complicated due to the requirement of meeting the three conditions, namely determination of the variation in time of the alcohol signal, measurement of the rate of flow of the exhaling air compared to a given value, and minimum maintenance of this value over a given time period, and it requires in addition corresponding monitoring (West German Offenlegungsschrift No. 26 10 578).